1. Field of the Invention
The present invention relates to a security element in the form of a tread or strip to be embedded in security documents such as bank notes, checks, bonds or the like, having characters, patterns, etc., that are readable by transmitted light to the naked eye and/or by machine, and to methods of producing such security documents.
2. Description of the Related Technology
It has been known for some time to protect bank notes and other money-value papers such as cheeks, shares, travelers checks, check cards and credit cards, but also passports and identity cards, by adding so-called security threads. Papers of this kind shall be referred to in the following as security papers. The security threads used for this purpose are made of a great variety of materials in the form of fine strips which are added in widths of 0.4 to 2 mm to the paper sheet during its manufacture. They are primarily produced from plastic films coated with metal foils, printed, colored or provided e.g. with pigmentlike substances. Furthermore, it is also known to give these security threads magnetic, fluorescent, X-ray absorbent and other properties by applying appropriate substances.
It has proven particularly advantageous to use security threads with microprints, whereby this microprint may contain information that is readable by the naked eye or else only by machine.
With respect to protection from forgery, security threads with metallically reflecting surfaces have proven particularly useful, since such threads introduced inside the paper are almost invisible by incident light, as the light rays passing through the upper layer of paper are completely reflected by the metallic surface and leave the paper diffusely scattered. By transmitted light, however, such threads appear as black strips which are clearly distinct from their surroundings. Threads of this kind cannot be imitated by printing on the paper. On the other hand, they show the disadvantage that a corresponding impression can be incorrectly aroused by introducing thin aluminum foils between two layers of paper. It has therefore been proposed to provide such aluminized security threads with microwriting (German laid open print no. 14 46 851). However, this has proven to be of little use in practice since this writing cannot be detected on the otherwise opaque security thread by transmitted light, and can only be detected with great difficulty by incident light. It is usually necessary to make the paper transparent with chemical means at least for the time of testing. The execution of such microwriting in special, e.g. fluorescent, colors has proven to be of little advantage in the same way and for the same reasons.
German "Auslegeschrift" no. 22 05 428 discloses a security thread designed as a metal strip and having a machine readable and/or visually readable coding applied by laser. The writing on this thread, that may possibly contain alphanumeric characters, consists of perforations, whereby the diameter of the holes or "line width" is to be small compared to the thickness of the metal strip in order to impede imitation.
Due to their extremely small line width, the characters of this security thread are detectable as poorly visible lighter areas on the dark background. Furthermore, such writing is relatively troublesome to produce, since very powerful lasers are required to provide the perforations in the metal thread. The perforation of the security thread material is so time-consuming that this method cannot be used for producing large amounts (thousands of kilometers) simply for reasons of time and thus of costs.
U.S. Pat. No. 4,652,015 also discloses a security thread to be used for bank notes and the like, on which metallically shining microcharacters are provided on clear transparent film material. The security thread and microcharacters located thereon as described in this patent are not detectable by incident light. However, by transmitted light solely the characters are detectable as sharp contours since the security thread itself is of transparent design. This security thread is produced by metalizing a transparent film over a large surface with a thin aluminum layer, printing the microcharacters on this layer using an acidproof ink and then etching away the unprinted areas, whereupon the characters are left on a transparent base.
By transmitted light an observer thus sees only writing that runs through the bank note. However, a disadvantage is that this writing is difficult to find due to the small size of the characters, its embedding in the paper pulp and a printed pattern that may be superimposed. The thread as such cannot be detected by the naked eye and cannot be felt as an uneven formation on the surface of the paper due to the extremely thin design required here. These detection features that normally characterize a security thread are thus no longer present in the known thread.